1. Field of the Invention
The present invention relates to an improvement in overshoot characteristics of a voltage regulator.
2. Description of the Related Art
A conventional voltage regulator includes, as illustrated in a block diagram of FIG. 11, a reference voltage circuit 1101, voltage dividing resistors 1103 and 1104, an error amplifier 1105, and an output transistor 1106.
The reference voltage circuit 1101 outputs a reference voltage VREF. The voltage dividing resistors 1103 and 1104 output a divided voltage VA by dividing an output voltage VOUT at an output terminal 1102 of the voltage regulator. The error amplifier 1105 amplifies a difference voltage between the reference voltage VREF and the divided voltage VA, and controls a gate of the output transistor 1106 by an output voltage VERR thereof. The output transistor 1106 maintains the output voltage VOUT at the output terminal 1102 constant based on a power supply voltage VDD of a voltage source 1107 and the output voltage VERR. The voltage regulator operates with the power supply voltage VDD.
The voltage regulator operates as follows to maintain the output voltage VOUT constant (see, for example, Japanese Patent Application Laid-open No. Hei 4-195613 (pages 1-3, FIG. 2).
When the output voltage VOUT decreases, the output voltage VERR of the error amplifier 1105 decreases to increase a gate-source voltage of the output transistor 1106. Accordingly, an ON-state resistance of the output transistor 1106 decreases to increase the output voltage VOUT. When the output voltage VOUT increases, the output voltage VERR of the error amplifier 1105 increases to decrease the gate-source voltage of the output transistor 1106. Accordingly, the ON-state resistance of the output transistor 1106 increases to decrease the output voltage VOUT.
The conventional voltage regulator, however, has a problem in that an overshoot occurs in the output voltage VOUT when the power supply voltage VDD changes in a pulse manner in situations such as power on and power supply fluctuations or when a load connected to the output terminal 1102 of the voltage regulator abruptly changes.